The present invention relates to the detection of a ground fault in the rotor winding of a rotating machine in order to produce a warning in time to prevent damage to the machine.
In large rotating machines, the rotor winding, which is isolated from ground, is supplied with a direct current from an exciter. The rotor winding is carried by a rotor body which is itself grounded and the winding is enclosed by insulation which electrically isolates it from the rotor body. Eventually, the resistance of the winding insulation to ground will decrease to an extent which produces a low resistance path between the rotor winding and ground.
Since the rotor winding is electrically floating, the occurrence of a ground condition at one point of the rotor winding will not, by itself, result in damage since there is no return path for current flowing to ground. However, if a second such defect should appear, current will be shunted from a portion of the rotor winding to the rotor body and if this current is of sufficient magnitude, it can result in arcing and localized heating which can severely damage the rotor structure.
For the above reasons, large rotating machines are typically equipped with ground leakage alarm equipment for monitoring such rotor winding insulation faults. This equipment can take a variety of forms. One known system of this type includes brushes which are periodically brought into contact with the rotor shaft. The existence of a single ground fault is detected by imposing a low voltage signal on the winding and the rotor. The existence of a ground fault creates a closed current loop which can be detected.
In another type of system for this purpose, a voltage monitoring unit is coupled to the ends of the rotor winding via slip rings and is operated to provide indications of the voltages between ground and each end of the winding. An arrangement of this type is disclosed, for example, in U.S. Pat. No. 4,812,751.
In various other known systems, test voltages are applied periodically to a machine winding, such as disclosed in U.S. Pat. Nos. 3,887,867, 4,187,525 and 4,556,946.
In all of the above-described systems, with the exception of those which employ a magnet coupling, the monitoring system is connected to the rotor winding by brushes or slip rings which are themselves subject to wear, and are hence not highly reliable. Moreover, all of the known systems are relatively expensive and complex.